Chengen Du wrote: > Hi, > > I would like to provide some additional explanations about the patch. > > > On Sat, Jun 8, 2024 at 10:54 AM Chengen Du <chengen.du@xxxxxxxxxxxxx> wrote: > > > > The issue initially stems from libpcap. The ethertype will be overwritten > > as the VLAN TPID if the network interface lacks hardware VLAN offloading. > > In the outbound packet path, if hardware VLAN offloading is unavailable, > > the VLAN tag is inserted into the payload but then cleared from the sk_buff > > struct. Consequently, this can lead to a false negative when checking for > > the presence of a VLAN tag, causing the packet sniffing outcome to lack > > VLAN tag information (i.e., TCI-TPID). As a result, the packet capturing > > tool may be unable to parse packets as expected. > > > > The TCI-TPID is missing because the prb_fill_vlan_info() function does not > > modify the tp_vlan_tci/tp_vlan_tpid values, as the information is in the > > payload and not in the sk_buff struct. The skb_vlan_tag_present() function > > only checks vlan_all in the sk_buff struct. In cooked mode, the L2 header > > is stripped, preventing the packet capturing tool from determining the > > correct TCI-TPID value. Additionally, the protocol in SLL is incorrect, > > which means the packet capturing tool cannot parse the L3 header correctly. > > > > Link: https://github.com/the-tcpdump-group/libpcap/issues/1105 > > Link: https://lore.kernel.org/netdev/20240520070348.26725-1-chengen.du@xxxxxxxxxxxxx/T/#u > > Fixes: 393e52e33c6c ("packet: deliver VLAN TCI to userspace") > > Cc: stable@xxxxxxxxxxxxxxx > > Signed-off-by: Chengen Du <chengen.du@xxxxxxxxxxxxx> > > --- > > net/packet/af_packet.c | 57 ++++++++++++++++++++++++++++++++++++++++-- > > 1 file changed, 55 insertions(+), 2 deletions(-) > > > > diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c > > index ea3ebc160e25..8cffbe1f912d 100644 > > --- a/net/packet/af_packet.c > > +++ b/net/packet/af_packet.c > > @@ -538,6 +538,43 @@ static void *packet_current_frame(struct packet_sock *po, > > return packet_lookup_frame(po, rb, rb->head, status); > > } > > > > +static u16 vlan_get_tci(struct sk_buff *skb) > > +{ > > + struct vlan_hdr vhdr, *vh; > > + u8 *skb_orig_data = skb->data; > > + int skb_orig_len = skb->len; > > + > > + skb_push(skb, skb->data - skb_mac_header(skb)); > > + vh = skb_header_pointer(skb, ETH_HLEN, sizeof(vhdr), &vhdr); > > + if (skb_orig_data != skb->data) { > > + skb->data = skb_orig_data; > > + skb->len = skb_orig_len; > > + } > > > The reason for not directly using skb_header_pointer(skb, > skb_mac_header(skb) + ETH_HLEN, ...) to get the VLAN header is due to > the check logic in skb_header_pointer. In the SOCK_DGRAM and > PACKET_OUTGOING scenarios, the offset can be a negative number, which > causes the check logic (i.e., likely(hlen - offset >= len)) in > __skb_header_pointer() to not work as expected. The calculation is still correct? I think that this is not the first situation where negative offsets can be given to skb_header_pointer. > While it is possible to modify __skb_header_pointer() to handle cases > where the offset is negative, this change could affect a wider range > of code.
